Fluorescence microscopic characterization of ionic polymer bead-supported phospholipid bilayer membrane systems

Supported phospholipid membrane structures on cationic organic polymer beads were prepared using mixtures of dioleoylphosphatidylserine (PS) and egg yolk phosphatidylcholine (PC). Confocal fluorescence microscopic observations using a fluorescent membrane probe (N-4-nitrobenzo-2-oxa-1,3-diazole-phosphatidylethanolamine) revealed that the phospholipid molecules in the PS/PC-bead complexes were along the outer surface of the beads, but not inside the beads. The anionic PS on the most outer surface of the PS/PC-bead complexes was responsible for the binding of a positively charged macromolecule, rhodamine isothiocyanate dextran (Mw 70,000) by electrostatic attractive forces. The fluidity of the membranes in the PS/PC-bead complexes was investigated by the fluorescence recovery after a photobleaching technique. The lateral diffusion coefficients (D) for the PS/PC-bead complexes were one-half or less than that for 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine giant unilamellar vesicles without solid supporting materials. Such a constrain of the phospholipid bilayer membrane in the complexes appeared to be due to its immobilization on the cationic polymer bead by electrostatic attractive forces between the PS and ammonium group on the surface of the bead. The D values for the complexes were dependent on the phospholipid composition; the PS(25 mol%)/PC(75 mol%)-bead complex produced a more fluid membrane than the PS(50 mol%)/PC(50 mol%)-bead one. Thus, the fluidity of the phospholipid bilayer membranes formed on the cationic polymer beads was significantly affected by the anionic phospholipid fraction used for the preparation of the complexes.